The present invention generally relates to stencil masks and charged particle beam exposure methods and apparatuses, and more particularly to a stencil mask having blanking electrodes and charged particle beam exposure method and apparatus which expose patterns using such a stencil mask.
Recently, the integration density of integrated circuit devices is becoming more and more improved. As a result, the photolithography technique which was conventionally used for forming patterns is being replaced by charged particle beam exposure methods, X-ray exposure methods and the like. The electron beam exposure method in particular is one of the promising charged particle beam exposure methods because fine patterns on the order of microns or even less can be formed by use of the electron beam.
The resolution of the electron beam exposure is high and fine patterns can be formed using the electron beam exposure method. However, there is a limit to the processing capability of the electron beam exposure method because the patterns are drawn by the stroke of the electron beam.
Various proposals have been made to improve the processing capability of the electron beam exposure method. For example, there is a proposed method which forms in advance sequences for generating patterns such as rectangular, square and triangular patterns using a variable rectangular aperture and selecting one of the sequences to irradiate and expose a desired pattern. A Japanese Laid-Open Patent Application No. 52-119185 proposes an exposure method using a mask in which the patterns are arranged in blocks. On the other hand, a Japanese Laid-Open Patent Application No. 62-260322 proposes an exposure method using a plate which includes rectangular apertures for forming general rectangular patterns and apertures for forming repeating patterns which are required to form a memory cell or the like.
The method of forming the repeating patterns is also referred to as a block exposure method or a cell production method, and is an effective method when forming a device such as a memory in which the repeating patterns occupy a large portion of the area to be exposed. Suitable ones of the repeating patterns are formed as block patterns, and one block pattern region is selected. The charged particle beam is irradiated on the selected block pattern region, and the charged particle beam transmitted through the selected block pattern region is repeatedly irradiated at desired positions on a wafer.
FIG. 1 generally shows an example of an electron beam exposure apparatus which performs the block process. The electron beam exposure apparatus is made up of an exposure part 10 and a control part 50. The exposure part 10 includes a charged particle beam generating source 14, a first slit 15, a first electron lens 16, a slit deflector 17, second and third lenses 18 and 19, a transmission (stencil) mask 20, first through fourth deflectors 21 through 24, blanking electrodes 25, a fourth lens 26, an aperture 27, a refocusing coil 28, a fifth lens 29, a dynamic focusing coil 30, a dynamic stigmator coil (i.e., an astigmatic correction coil) 31, a sixth lens 32, electromagnetic main deflection coils 33, electrostatic sub deflection means 34, a stage 35, and first through fourth alignment coils 36 through 39.
The charged particle beam generating source 14 includes a cathode electrode 11, a grid electrode 12 and an anode electrode 13. The first slit 15 forms the cross section of the charged particle beam into a rectangular shape, for example, and the first electron lens 16 converges the shaped charged particle beam. The slit deflector 17 deflects and corrects the beam position depending on a correction deflection signal S1. The second and third lenses 18 and 19 confront each other, and the stencil mask 20 is arranged between the second and third lenses 18 and 19 and is movable in the horizontal direction.
The first through fourth deflectors 21 through 24 are arranged above and below the stencil mask 20 and deflect the beam between the second and third lenses 18 and 19, depending on respective position information P1 through P4, so as to select one of the transmission holes formed in the stencil mask 20. The blanking electrodes 25 block or transmit the beam depending on a blanking signal SB. The electromagnetic main deflection coils 33 and the electrostatic sub deflection means 34 position the beam on a wafer W which is placed on the stage 35 depending on exposure position determination signals S2 and S3. The stage 35 is movable in the X and Y directions.
On the other hand, the control part includes a recording medium, a central processing unit (CPU) 52, an interface 53, a data memory 54, a pattern controller 55, an amplifier part 56, a mask moving mechanism 57, a blanking control circuit 58, an amplifier part 59, a sequence controller 60, a stage control mechanism 61, a laser interferometric device 62, a deflection control circuit 63, and amplifier parts 64 and 65.
The recording medium 51 stores design data related to integrated circuit devices. The CPU 52 controls the entire exposure apparatus. The interface 53 transfers various information which are entered by the CPU 52, such as drawing pattern information related to a pattern which is to be drawn, drawing position information related to a position on the wafer W where the pattern is to be drawn and mask information related to the stencil mask 20. The data memory 54 stores the drawing pattern information and the mask information which are transferred from the interface 53.
The pattern controller 55 functions as a specifying means, a holding means, a calculation means and an output means for carrying out various processes. For example, the pattern controller 55 specifies one of the transmission holes of the stencil mask 20, depending on the drawing pattern information and the mask information, and generates position data indicative of a position of the specified transmission hole on the stencil mask 20. In addition, the pattern controller 55 calculates a correction value H which is dependent on the difference between the respective shapes of the pattern which is to be drawn and the specified transmission hole.
The amplifier part 56 generates the correction deflection signal S1 from the correction value H. The mask moving mechanism 57 moves the stencil mask 20, if necessary, in response to a move signal MS from the pattern controller 55. The blanking control circuit 58 receives a blanking control signal BL from the pattern controller 55, and the amplifier part 59 generates the blanking signal SB based on an output signal of the blanking control circuit 58. The sequence controller 60 controls the drawing process sequence depending on the drawing position information which is transferred from the interface 53. The stage control mechanism 61 moves the stage 35 if necessary. The laser interferometric device 62 detects the position of the stage 35. The deflection control circuit 63 calculates an exposure position on the wafer W. The amplifier parts 64 and 65 respectively generate the exposure position determination signals S2 and S3.
For example, the stencil mask 20 is made of a silicon wafer. FIG. 2 is a plan view showing a mask part of the stencil mask 20.
As shown in FIG. 2, a plurality of areas are formed in a matrix arrangement at predetermined intervals on the stencil mask 20. In the example shown, nine areas E1 through E9 are formed in a matrix arrangement on the stencil mask 20 at a predetermined pitch EL. Each of the areas E1 in each of the X- and Y- directions, or coordinates through E9 has a size corresponding to a maximum deflection range of the beam on the stencil mask 20, and each side of one square area is of a common value, selected from the range of approximately 1 to 5 mm. In other words, when the beam is irradiated within the area E1, for example, the beam can be deflected within the area E1 using the deflectors 21 through 24, but the stencil mask 20 must be moved if the beam is to be irradiated within the area E9, for example. A reference point is located at the left bottom corner of each of the areas E1 through E9 as indicated by a black circular mark, and XY coordinate values are assigned to the reference point of each area. For example, an area coordinate E.sub.xy =(1, 1) describes the area E7.
As shown in FIG. 3, a plurality of blocks are formed in a matrix arrangement at predetermined intervals within each of the areas E1 through E9. In the example shown, thirty-six blocks B.sub.1 through B.sub.36 are formed in a matrix arrangement at a predetermined pitch BL within the area E1. The size of one block corresponds to the size of the beam on the stencil mask 20, that is, the cross section of the beam on the stencil mask 20. For example, each side of one square block is of a common value, selected from the range of approximately 100 to 500 .mu.m. The XY coordinate values are also assigned to a reference point of each block. For example, a block coordinate B.sub.xy =(1, 2) describes the block B.sub.32.
In other words, an arbitrary block within an arbitrary area within the stencil mask 20 can be described by the area coordinate E.sub.xy and the block coordinate B.sub.xy. When E.sub.xy =(1, 1) and B.sub.xy =(1, 2), for example, the block B.sub.32 within the area E7 is specified. In order to carry out the exposure efficiently, it is desirable that all of the patterns required to expose one layer of one particular kind of large scale integrated circuit (LSI) are formed within one area of the stencil mask 20. In FIG. 3, the blocks B.sub.1, B.sub.6, B.sub.31 and B.sub.36 which are indicated by hatchings denote transmission holes which are used when forming variable rectangular patterns.
FIG. 4 shows an example of contact hole patterns of a dynamic random access memory (DRAM). A plurality of memory elements are distributed in the DRAM, and the contact holes are arranged in correspondence with each of the memory elements as shown. When a region surrounded by a solid line 71 is regarded as a block, for example, it is possible to expose the contact hole patterns of the DRAM which are distributed on a plane by repeating the exposure of the block pattern. In the example shown, thirty contact holes are included in one block, and the entire region can be exposed by carrying out the exposure a number of times equal to 1/30th of the number of memory elements. In other words, the block exposure method achieves a high-speed exposure with respect to regular or repeating patterns.
FIG. 5 shows an example of patterns of a plurality of blocks formed within one area of the stencil mask 20. FIG. 5 shows only four kinds of patterns, but the shapes of the repeating patterns are not so limited.
As described above, the block exposure method has an advantage in that the exposure of repeating patterns can be made at a high speed. However, not all LSI patterns can be exposed at a high speed using the block exposure method. Unless exactly the same pattern repeats, the high-speed exposure of the block exposure method cannot be realized because the irradiating position of the beam on the stencil mask 20 must be shifted from one area to another area which includes the desired patterns and this shift can only be made by moving the stencil mask 20. But it takes time to move and accurately position the stencil mask 20.
on the other hand, the contact holes of the ROM, for example, are regularly formed to a certain extent. However, the positions of the contact holes are not perfectly regular, and the contact holes cannot be formed by simply exposing exactly the same pattern repeatedly. As a result, it is necessary to move the stencil mask 20 so that the irradiating position of the beam on the stencil mask 20 is shifted from one area to another area which includes the desired patterns. Furthermore, when the number of areas which include similar but not identical patterns increases, it becomes necessary to change the stencil mask 20 to another different stencil mask in order to expose all of the desired patterns of an integrated circuit device because only a limited number of areas can be provided on the stencil mask 20. But it takes a considerably long time to change the old stencil mask and correctly position the new stencil mask.
There is also an exposure method which uses a blanking aperture array. The blanking aperture array includes apertures which are arranged two-dimensionally throughout the entire region of the blanking aperture array and each is provided with a corresponding pair of blanking electrodes. A voltage is applied across a pair of blanking electrodes of an aperture when the electron beam transmitted through this aperture is to be deflected outside an exposure region on a wafer. Hence, by controlling the supply of the voltage to each of the blanking aperture pairs of the blanking aperture array, it is possible to expose arbitrary fine patterns. An example of such an exposure method using the blanking aperture array is proposed in a Japanese Published Utility Model Application No. 56-19402.
For example, each square aperture of the blanking aperture array has a side of 7 .mu.m, and the apertures are arranged with a pitch of 3 .mu.m when exposing a square pattern having a side of 0.07 .mu.m. The pitch is smaller than the side of the aperture so that patterns of the adjacent apertures are exposed on the wafer connect. In other words, the aperture itself is extremely small and the pitch with which the apertures are arranged is also extremely small, so that arbitrary patterns can be exposed with a high resolution. But because of the extremely small size and pitch of the apertures, it is very difficult technically, if not impossible, to form the necessary interconnections for the blanking electrode pairs on the blanking aperture array because of the extremely small pitch with which the apertures are arranged.